Some might just "complain", but others feel safer being vocal in a group of people.
You mention understanding that some of the quietest people have the most to say behind a screen, well, I imagine there is a non-insignificant group of people who feel much safer talking it out within a group where there isn't a written record and they can judge the crowd's reaction immediately.
I wouldn't consider myself anywhere close to an extrovert and I would consider myself more candid in spoken communications at work, even when my whole team is present.
Just my 2 cents.